1. Field of the Invention
The present invention relates to an aspherical objective lens for use in an optical system of an optical pickup of an optical recording and reproducing apparatus.
2. Description of Related Art
Among optical recording and reproducing apparatuses, available is an optical disc player for reading information recorded on optical discs such as optical recording medium, that is, LD (laser disc), CD (compact disc), CD-R (compact disc recordable), and DVD (digital video disc). Also available is a compatible disc player for reading information from a plurality of optical discs.
The optical pickup used for these is provided with an optical system that irradiates an optical disc with an optical beam to read the return beam from the optical disk. This optical disc system is designed to different specifications such as numerical aperture NA, the thickness of an optical disc substrate, and an optimum optical wavelength for reading. Therefore, it is necessary to correct differences at least in the aforementioned numerical aperture NA and thickness in order to implement an optical pickup for an LD/CD/DVD compatible player. The optical pickup schemes of the compatible player are explained below in (1) through (3).
This scheme requires at least two objective lenses for one pickup and a mechanism for switching between these lenses, thereby hindering a reduction in size and cost of the pickup.
This scheme allows for reproducing optical discs, different in thickness, using zero-order and first-order diffracted beams of light of the hologram. However, one diffracted beam of light always occurs even when the other diffracted beam of light is being reproduced and a diffracted beam of light other than the aforementioned beams occurs in practice even in a small amount, thereby providing a low efficiency of the optical power. In addition, the addition of a hologram increases the number of parts involved, thus hindering a reduction in the size and cost of the pickup. Moreover, a hologram can be formed on a plane of an objective lens with the hologram and the objective lens integrated to provide a reduced size and low cost, however, there is no change in the fact that an unnecessary beam of light occurs even when the hologram is integrated. Moreover, such objective lenses provide poor mass productivity compared with typical aspherical objective lenses.
This scheme allows for using the inner and outer circumference portions of an orbicular zone to reproduce an optical disc of a thin substrate, while using the inner and middle circumference portions to reproduce an optical disc of a thick substrate.
The use of such an objective lens allows one objective lens to record and reproduce optical discs having different specifications, for example, the thickness of the substrate, without adding, in particular, additional parts to the optical system. However, this would provide the objective lens with a step, thus making the shape of the lens discontinuous. This would therefore require workmanship of a higher level than that for a conventional aspherical lens, providing poor mass productivity and increasing the cost of the pickup.
Objective lenses for use in the pickup of an optical disc are designed to the thickness of the optical disc to be reproduced. Reproducing an optical disc having a thickness different from the thickness for which the disc has been designed would not allow for forming an adequate spot on the optical disc due to spherical aberration. Consequently, an adequate reproducing performance cannot be obtained. The amount of spherical aberration caused by a difference in thickness between the optical disc and the designed value therefor is proportional to the fourth power of the numerical aperture of the objective lens. Therefore, decreasing the numerical aperture by adding an aperture to the optical system allows for reducing the spherical aberration. On the other hand, the diameter of a spot on the optical disc is inversely proportional to the numerical aperture of the objective lens. Decreasing the numerical aperture can reduce the aberration, but also causes the diameter of the spot to increase.
Therefore, no excessive decrease in the numerical aperture is allowed. Specifications have been established as in Table 1 for DVD, CD, and CD-R, which are currently in vogue for optical discs.
As shown in the table, CD and CD-R have a smaller numerical aperture than that of DVD, and thus it is conceivable to limit the aperture so that the objective lens of DVD has a numerical aperture of 0.45. However, practically as shown by curve B in FIG. 1, a numerical aperture of 0.45 set by the aperture to reproduce a CD (particularly, CD-R) by the objective lens for the DVD would not provide a good reproducing performance since the aberration is 0.07xcex or greater, that is, greater than Marechal""s limit. Actual calculation of the diameter of the spot based on the wavefront aberration teaches, as shown in FIG. 2, that a side lobe increases significantly compared with the case of a CD-dedicated objective lens.
Prior art scheme (3) is available as the most simplified method for solving such problems, however, such a lens as has the shape of special orbicular recessed portions provides inefficient mass productivity.
In view of the aforementioned problems, the object of the present invention is to provide an aspherical objective lens having performance equivalent to the aforementioned scheme (3) without forming orbicular recessed portions thereon and making the shape of the objective lens discontinuous.
The aspherical objective lens, according to the present invention, is an objective lens capable of converging first and second beams of light of first and second wavelengths individually on a first optical disc having a first substrate thickness and on a second optical disc having a second substrate thickness greater than the first substrate thickness at approximately a valid first numerical aperture and a second numerical aperture less than the first numerical aperture, respectively; characterized in that a difference between the average values of variations in wavefront aberration corresponding to an inner circumference portion and an outer circumference portion, when said first beam of light converges on said first optical disc having said first substrate thickness, is approximately equal to one wavelength of a beam of light of said first wavelength.
The aspherical objective lens, according to the present invention, is characterized in that wavefront aberration corresponding to numerical apertures up to a middle circumference portion between said inner and outer circumference portions is equal to or less than Marechal""s limit when said second beam of light is converged on said substrate of the second thickness.
The method for manufacturing the objective lens, according to the present invention, is a method for manufacturing an aspherical objective lens capable of converging first and second beams of light of first and second wavelengths individually on a first optical disc having a first substrate thickness and on a second optical disc having a second substrate thickness greater than the first substrate thickness at approximately a valid first numerical aperture and a second numerical aperture less than the first numerical aperture, respectively,
said method comprising the steps of setting parameters for specifying a lens plane of a target lens, having an inner circumference portion around an optical axis, which allows a beam of light of said first wavelength to converge almost with no aberration on a transparent substrate having an assumed substrate of a thickness between said first substrate thickness and the second substrate thickness at a third target numerical aperture approximately equal to or less than said second numerical aperture;
calculating an inner circumference average value of variations in a target wavefront aberration corresponding to the inner circumference portion of said target lens in accordance with said parameters when a beam of light of said first wavelength is allowed for converging on an optimal image plane on said first optical disc; and
varying and updating said parameters so that the outer circumference portion of said target lens converges almost with no aberration on the transparent substrate having said substrate of the first thickness, and an average value of variations in the wavefront aberration thereof occurs at a position apart by one wavelength of a beam of light of said first wavelength from said inner circumference average value.
The aspherical objective lens and its manufacturing method are characterized in that said second wavelength is longer than said first wavelength.
According to the present invention, aberration is equal to or less than 0.07xcex rms for both optical discs of a thin substrate thickness, that is, the first substrate thickness and of a thick substrate thickness, that is, the second substrate thickness in the middle circumference portion. The aberration is corrected on the thin substrate in the outer circumference portion by a phase difference of approximately one wavelength relative to the inner circumference portion. Wavefronts of the inner and outer circumference portions are thus allowed for being joined smoothly with no discontinuity in the middle circumference portion. This allows the objective lens to take a continuous shape, thereby providing high-mass productive of objective lenses for use in compatible disc players.